Reverse osmosis is a process known in the art to supply water to parts of the world where access to seawater is plentiful, but where there is little fresh water.
Reverse osmosis is a process, which uses a force that is in reverse of the normal osmotic pressure to force a solution containing a solute (e.g., seawater) through a semi-permeable membrane. This process has the effect of splitting the solute stream into a permeate stream and a waste stream. The permeate stream has a very low salt content and is typically potable. The waste stream has a higher concentration of salt than the solute and is known as “concentrate.”
The reverse osmosis process requires substantial energy to separate the solute into a permeate stream and a concentrate stream using semi-permeable membranes. This energy is primarily required to power high-pressure pumps that are used to drive fluids through the membranes.
A work exchanger is an energy recovery device known in the art which is used to reduce the net energy required by the reverse osmosis process by recovering the potential (pressure) energy contained in the concentrate leaving the reverse osmosis (semi-permeable) membrane module. The amount of potential energy contained in the concentrate stream is typically sixty percent (60%) of the total energy required by the reverse osmosis process when applied to a solute such as seawater.
Work exchanger energy recovery devices have the potential to increase efficiency by recovering as much as ninety-eight percent (98%) of the potential energy contained in the concentrate stream.
A work exchanger system is used to make the process of recovering the energy from the concentrate stream continuous by utilizing pairs of pressure vessels operating in an appropriate sequence.
A typical work exchanger system utilizes both vessels and tanks. Tanks are low-pressure devices which are filled with seawater. Tanks may be elevated or pressurized. Elevated tanks are essentially at atmospheric pressure (1 bar) but because they are elevated there is a head (pressure) in the process equipment located below the tank, which is below the elevated tank. If the seawater level is kept constant in the elevated tank then the head (pressure measured in meters or feet of “water column”) will remain constant; small changes in fluid level will cause minor changes in the head.
Pressurized tanks “simulate” the effect of elevation by using a pump (e.g., assisted by an air or nitrogen filled bladder) which creates as continuous a pressure as possible in the tank. “Pressure” and “head” are directly related concepts known in the art.
Vessels are components utilized in work exchangers. There are essentially two types of vessels for reverse osmosis processes that utilize work exchangers: the membrane pressure vessel and the work exchanger pressure vessel. A membrane pressure vessel contains the reverse osmosis membrane. A work exchanger pressure vessel contains solute and concentrate and may contain an interface (septum) for separating solute and concentrate. Work exchanger vessels known in the art are pressurized and depressurized with a maximum pressure as required by the reverse osmosis membranes to approximately 70 bar.
The process of recovering the energy from the concentrate stream is achieved by directing the concentrate stream directly against the low-pressure solute which is about to be desalinated immediately before it contacts the membranes of the reverse osmosis component of the desalination device. This is accomplished by placing a vessel filled with solute at atmospheric or slightly above atmospheric pressure in contact with the concentrate stream that is at high-pressure. The high-pressure is transferred virtually instantly to the low-pressure solute that becomes pressurized to the same level as the high-pressure concentrate stream. This process is made continuous by a work exchanger system, typically comprised of pairs of pressure vessels operating in an appropriate sequence.
Each pressure vessel used during the two-stroke cycle has at least two ports: a concentrate port at one end, and a solute port at the other end. Each pair of vessels may further include a component (referred to herein as a “septum”) which freely slides between the ports (or, alternatively, the interface between the high-pressure and low-pressure fluids may serve as the septum). A system of valves connect and disconnect the concentrate ports to a high-pressure waste stream concentrate line, a low-pressure discharge concentrate line, a low-pressure solute (feed) line and a high-pressure solute (feed) line.
At the first stroke, the concentrate port is connected to the high-pressure concentrate line, while the feed port is connected to the high-pressure feed line. The vessel is filled with high-pressure concentrate that displaces the septum back toward the feed port to direct feed into the high-pressure feed line and toward the reverse osmosis membranes.
At the second stroke, the concentrate port is connected to the concentrate discharge line while the feed port is connected to the low-pressure solute feed line. The vessel is filled with low-pressure feed that displaces the septum towards the concentrate port and concentrate is discharged through the non- or low-pressurized discharge line.
The foregoing discussion describes a two-vessel, two-port embodiment, but other embodiments may include additional vessels or ports.
Valve design is critical to the operation of a work exchanger device. A typical work exchange system includes various configurations of valves which control the flow of pressurized solute, typically seawater, and concentrate through the reverse osmosis process and which are used to make the process continuous. Hereafter the discussion will focus on seawater as the solute.
Each of the vessels utilizes at least two types of valves: a seawater valve type and a concentrate valve type. The seawater valves are generally non-actuated check valves that open and close in response to the pressure and flow of concentrate through the actuated concentrate valves.
Publication WO/76639 “A Method and a Plant for Production of Fresh Water from Briny Water” teaches a system which does not require a physical septum to achieve separation between return-brine (concentrate) and fresh salt water (solute) without the use of a physical structure for a septum. Separation of these two fluids is accomplished without the use of a physical septum by controlling the flow of fluids through the work exchanger pressure vessels. Publication WO/76639 contemplates the use of sieve plates or screens to more evenly distribute flow through the vessel. However, Publication WO/76639 does not teach specific physical structures or geometry for designing a sieve plate.
Moreover, there are design limitations for systems which use horizontal vessels and a physical septum. The separation (“physical septum”) must be longer than the diameter of the pressure vessel to prevent binding or sticking in the vessel and therefore limits the diameter of the vessel. In order to obtain reasonable volume, the lack of diameter must be made up by increasing the length making the vessel exceptionally long; typically 21 feet long. When using a septum in a work exchanger vessel to separate the solute from the concentrate, it is necessary to provide a method of allowing the solute to pass through the septum at the end of the ‘fill’ stroke to accomplish what is known in the art as ‘overflush’. While overflush can be fine-tuned to the point of zero overflush, it is necessary to provide for the passage of solute through the septum or partitioning device to prevent stalling of the process and/or slamming of the septum against the end of the vessel. Similarly, it is necessary to provide a means to allow concentrate to pass through the septum in the ‘power’ stroke should the septum reach the opposite end of the vessel resulting in what is known in the art as ‘mixing’. While this is an undesirable condition under operational circumstances, it is unavoidable in the setting up and fine tuning of the device during startup of the reverse osmosis process and if no method is provided to allow the fluids involved in the process to pass through the septum, the system will stall and physical damage will result. Therefore, work exchangers employing a physical septum must incorporate a method of passing fluids through the septum in both directions resulting in additional manufacturing costs and potential malfunctions.
It is desirable to implement a work exchanger system that optimizes monitoring and control of the mixing interface thus creating an inherently reliable virtual septum, which eliminates the need for a physical septum.
It is further desirable to overcome limitations in the prior art requiring the use of very long vessels utilizing a physical septum, and thus allow more flexibility in the design of systems by enabling the use of shorter vessels of larger diameter.
It is further desirable to optimize a work exchanger system to provide synchronization and sealing of the valves in order to create a reliable virtual septum and efficiently perform the two-stroke cycle.
It is further desirable to measure and optimize the physical properties of the mixing interface.
It is further desirable to optimize the flow paths in and out of the flow paths through the nozzles and heads of the work exchanger.
It is further desirable to design multiple-orifice flow distribution plates or similar components which optimize the flow of fluid through work exchanger pressure vessels.
It is further desirable that the flow distribution devices (e.g., multiple orifice flow distribution plates) create the virtual septum in such a way as to minimize the volume the septum occupies (and by extension, the volumes of solute and concentrate required to create the virtual septum). The virtual septum thus created floats on top of the concentrate and the solute floats on top of the virtual septum.
It is further desirable to optimize the control of two-port valves by allowing control of the flow in/out of the vessels to be operated independently.
It is further desirable that the valves controlling the work exchanger vessels are operated in a sequence which allows the virtual septum to move back and forth between the flow distribution devices. The methods of controlling the valves are varied (e.g., timing, or sensing the position of the virtual septum) but the virtual septum should never be allowed to impinge upon a flow distribution device as this will require the virtual septum to reestablish itself (which may take several valve cycles).
It is desirable that an adequate supply of solute is available to ensure there is always adequate pressure and flow during the cyclic operation of the work exchangers. Unnecessary fluctuations in solute pressure or flow can compromise the stability of the virtual septum.
It is further desirable to augment the pumped volume of solute with one or more reservoirs of solute which are maintained at the appropriate pressure either by elevation or pumping (during the concentrate fill cycle of the work exchanger).